git-send-email: delay creation of MIME headers

After the next patch, git-send-email will sometimes modify
existing Content-Transfer-Encoding headers.  Delay the addition
of the header to @xh until just before sending.  Do the same
for MIME-Version, to avoid adding it twice.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1324,6 +1324,8 @@ sub send_message {
 	my $author_encoding;
 	my $has_content_type;
 	my $body_encoding;
+	my $xfer_encoding;
+	my $has_mime_version;
 	@to = ();
 	@cc = ();
 	@xh = ();
@@ -1394,9 +1396,16 @@ sub send_message {
 				}
 				push @xh, $_;
 			}
+			elsif (/^MIME-Version/i) {
+				$has_mime_version = 1;
+				push @xh, $_;
+			}
 			elsif (/^Message-Id: (.*)/i) {
 				$message_id = $1;
 			}
+			elsif (/^Content-Transfer-Encoding: (.*)/i) {
+				$xfer_encoding = $1 if not defined $xfer_encoding;
+			}
 			elsif (!/^Date:\s/i && /^[-A-Za-z]+:\s+\S/) {
 				push @xh, $_;
 			}
@@ -1444,10 +1453,9 @@ sub send_message {
 		if defined $cc_cmd && !$suppress_cc{'cccmd'};
 
 	if ($broken_encoding{$t} && !$has_content_type) {
+		$xfer_encoding = '8bit' if not defined $xfer_encoding;
 		$has_content_type = 1;
-		push @xh, "MIME-Version: 1.0",
-			"Content-Type: text/plain; charset=$auto_8bit_encoding",
-			"Content-Transfer-Encoding: 8bit";
+		push @xh, "Content-Type: text/plain; charset=$auto_8bit_encoding";
 		$body_encoding = $auto_8bit_encoding;
 	}
 
@@ -1467,14 +1475,19 @@ sub send_message {
 				}
 			}
 			else {
+				$xfer_encoding = '8bit' if not defined $xfer_encoding;
 				$has_content_type = 1;
 				push @xh,
-				  'MIME-Version: 1.0',
-				  "Content-Type: text/plain; charset=$author_encoding",
-				  'Content-Transfer-Encoding: 8bit';
+				  "Content-Type: text/plain; charset=$author_encoding";
 			}
 		}
 	}
+	if (defined $xfer_encoding) {
+		push @xh, "Content-Transfer-Encoding: $xfer_encoding";
+	}
+	if (defined $xfer_encoding or $has_content_type) {
+		unshift @xh, 'MIME-Version: 1.0' unless $has_mime_version;
+	}
 
 	$needs_confirm = (
 		$confirm eq "always" or